[Bug tree-optimization/105740] missed optimization switch transformation for conditions with duplicate conditions

["marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105740

--- Comment #9 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to luoxhu from comment #8)
> (In reply to rguenther@suse.de from comment #6)
> > On Tue, 21 Jun 2022, jakub at gcc dot gnu.org wrote:
> > 
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105740
> > > 
> > > --- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> > > The problem with switch-conversion done multiple times is that when it is done
> > > early, it can do worse job than when it is done late, e.g. we can have better
> > > range information later which allows (unfortunately switch-conversion doesn't
> > > use that yet, there is a PR about it) to ignore some never reachable values
> > > etc.
> > > So ideally we either need to be able to undo switch-conversion and redo it if
> > > things have changed, or do it only late and for e.g. inlining costs perform it
> > > only in analysis mode and record somewhere what kind of lowering would be done
> > > and how much it would cost.
> > > With multiple if-to-switch, don't we risk that we turn some ifs into switch,
> > > then
> > > switch-conversion lowers it back to ifs and then another if-to-switch matches
> > > it again and again lowers it?
> > 
> > Yeah, I think ideally switch conversion would be done as part of switch
> > lowering (plus maybe an extra if-to-switch).  The issue might be what
> > I said - some passes don't like switches, but they probably need to be
> > taught.  As of inline cost yes, doing likely-switch-converted analysis
> > would probably work.
> 
> git diff
> diff --git a/gcc/passes.def b/gcc/passes.def
> index b257307e085..1376e7cb28d 100644
> --- a/gcc/passes.def
> +++ b/gcc/passes.def
> @@ -243,8 +243,6 @@ along with GCC; see the file COPYING3.  If not see
>          Clean them up.  Failure to do so well can lead to false
>          positives from warnings for erroneous code.  */
>        NEXT_PASS (pass_copy_prop);
>        /* Identify paths that should never be executed in a conforming
>          program and isolate those paths.  */
>        NEXT_PASS (pass_isolate_erroneous_paths);
> @@ -329,6 +327,7 @@ along with GCC; see the file COPYING3.  If not see
>        POP_INSERT_PASSES ()
>        NEXT_PASS (pass_simduid_cleanup);
>        NEXT_PASS (pass_lower_vector_ssa);
> +      NEXT_PASS (pass_if_to_switch);
>        NEXT_PASS (pass_lower_switch);
>        NEXT_PASS (pass_cse_reciprocals);
>        NEXT_PASS (pass_reassoc, false /* early_p */);
> 
> Tried this to add the second if_to_switch before lower_switch, but switch
> lowering doesn't work same as switch_conversion:

Note the lowering expand to a decision tree where node of such tree can be
jump-tables,
bit-tests or simple comparisons.

> 
> ;; Function test2 (test2, funcdef_no=0, decl_uid=1982, cgraph_uid=1,
> symbol_order=0)
> 
> beginning to process the following SWITCH statement ((null):0) : -------
> switch (_2) <default: <L27> [INV], case 1: <L20> [INV], case 2: <L21> [INV],
> case 3: <L22> [INV], case 4: <L2
> 3> [INV], case 5: <L24> [INV], case 6: <L25> [INV]>
> 
> ;; GIMPLE switch case clusters: JT(values:6 comparisons:6 range:6 density:
> 100.00%):1-6

So jump-table is selected. Where do you see this GIMPLE representation?

...

> 
> ASM still contains indirect jump table like -fno-switch-conversion:

> 
> Is this bug of lower_switch or expected?

What bug do you mean? 

> From the code, they have different
> purpose as switch_conversion turns switch to single if-else while

No switch_conversion expands a switch statement to a series of assignment
based on CSWITCH[index] arrays.

> lower_switch expand CLUSTERS as a decision tree.